The present invention relates to a denaturing method of a hydrophilic polymer and a producing method of a hydrophilic resin, and more particularly, to a producing method of a hydrophilic resin having excellent physical properties, such as an absorbing property, by uniformly denaturing a hydrophilic polymer, for example, by applying a crosslinking treatment to an absorbent resin.
Recently, an absorbent resin is used extensively in sanitary goods, such as paper diapers, sanitary napkins, and incontinence pads, to absorb body fluids. Besides the sanitary goods, the absorbent resin is also used extensively in water-retaining materials for soil to keep water in soil or drip absorbing materials to absorb drip from foods or the like.
Examples of such absorbent resins include: a partially neutralized crosslinked polymer of polyacrylic acid, a hydrolyzed graft polymer of starch-acrylonitrile, a neutralized graft polymer of starch-acrylic acid, a saponified copolymer of vinyl acetate-acrylic ester, a hydrolyzed copolymer of acrylonitrile or acrylamide, or a crosslinked product thereof, and a crosslinked polymer of a cationic monomer.
Notable properties of the absorbent resin include absorbency and absorbing rate when brought into contact with an aqueous liquid like a body fluid, liquid permeability, gel strength of swelled gel, and absorbing power of absorbing water from a base material containing an aqueous liquid, etc. However, each property is not necessarily correlated positively to one another. For example, the higher the absorbency, the lower the absorbing rate, liquid permeability, and gel strength. To solve the above problem, a crosslinking treatment (secondary crosslinking treatment) using a crosslinking agent (denaturant) is conventionally applied to the absorbent resin to well balance (improve) these properties and enhance the absorbing property.
A secondary crosslinking structure is introduced into the absorbent resin in the following manner: a swelling liquid in which the absorbent resin is swelled with a solvent, or a dispersing liquid in which the absorbent resin is dispersed in a dispersing medium is prepared, and a crosslinking agent or a solution thereof is added to the above prepared liquid and mixed with each other, so that the reaction of the absorbent resin and crosslinking agent takes place in a so-called solid-liquid system. The solvent or dispersing medium referred to herein is, for example, a hydrophilic compound, such as water and alcohol.
Examples of the above method of introducing the secondary crosslinking structure into the absorbent resin using a crosslinking agent include: a method of using polyhydric alcohol (Japanese Laid-open Patent Application No. 180233/1983 (Tokukaisho No. 58-180233) and Japanese Laid-open Patent Application No. 16903/1986 (Tokukaisho No. 61-16903)), a method of using a polyglycidyl compound, a polyaziridine compound, a polyamine, or polyisocyanate (Japanese Laid-open Patent Application No. 189103/1984 (Tokukaisho No. 59-189103)), a method of using glyoxal (Japanese Laid-open Patent Application No. 117393/1977 (Tokukaisho No. 52-117393)), a method of using polyvalent metal compound (Japanese Laid-open Patent Application No. 136588/1976 (Tokukaisho No. 51-136588), Japanese Laid-open Patent Application No. 257235/1986 (Tokukaisho No. 61-257235), and Japanese Laid-open Patent Application No. 7745/1987 (Tokukaisho No. 62-7745)), a method of using a silane coupling agent (Japanese Laid-open Patent Application No. 211305/1986 (Tokukaisho No. 61-211305), Japanese Laid-open Patent Application No. 252212/1986 (Tokukaisho No. 61-252212), and Japanese Laid-open Patent Application No. 264006/1986 (Tokukaisho No. 61-264006)), a method of using an epoxy compound and a hydroxy compound (Japanese Laid-open Patent Application No. 132103/1990 (Tokukaihei No. 2-132103)), a method of using alkylene carbonate (German Patent No. 4,020,780), etc.
Also, a variety of methods are proposed to distribute the crosslinking agent over the surface of the absorbent resin more evenly to crosslink near the surface of the absorbent resin uniformly. Examples of such methods are: a method of adding the crosslinking agent to the absorbent resin in the presence of inactive inorganic powders (Japanese Laid-open Patent Application No. 163956/1985 (Tokukaisho No. 60-163956) and (Japanese Laid-open Patent Application No. 255814/1985 (Tokukaisho No. 60-255814) ), a method of adding the crosslinking agent to the absorbent resin in the presence of dihydric alcohol (Japanese Laid-open Patent Application No. 292004/1989 (Tokukaihei No. 1-292004)), a method of adding the crosslinking agent to the absorbent resin in the presence of an ether compound (Japanese Laid-open Patent Application No. 153903/1990 (Tokukaihei No. 2-153903)), a method of adding the crosslinking agent to the absorbent resin in the presence of an alkylene oxide adduct of monohydric alcohol, an organic acid salt, or lactam (European Patent No. 555,692), etc.
However, the above conventional methods have the following problems. That is, because the conventional methods use the solvent or dispersing medium to react the absorbent resin with the crosslinking agent, these methods must include a post-treatment process, such as a removing step for removing the solvent or dispersing medium and a drying step. This is the reason why the entire procedure of introducing the secondary crosslinking structure into the absorbent resin is complicated. In addition, the crosslinking agent and the solvent or dispersing medium may reside in the post-reaction absorbent resin, thereby possibly making the resulting absorbent resin unsafe. Further, it is quite difficult to remove or collect an excessive crosslinking agent when the reaction ends.
Moreover, when microscopic powders of the absorbent resin are used, for example, a mixture of the absorbent resin and the solvent or dispersing medium produces an agglomerate, thereby making it impossible to swell or disperse the absorbent resin uniformly in a satisfactory manner. Thus, the secondary crosslinking structure can not be introduced into all kinds of absorbent resin uniformly because of their shape or size. In addition, the swelling liquid or dispersing liquid must be stirred relatively hard to let the absorbent resin swell or disperse the same uniformly in a satisfactory manner. Thus, the absorbent resin is susceptible to a physical damage, and. for example, the surface of the absorbent resin is often damaged when the reaction ends. Note that the agglomerate referred to herein means masses of agglomerated particles.
Also, to carry out the crosslinking treatment at a relatively high degree, for example, to attain high crosslinking density and depth, a relatively large amount of solvent or dispersing medium must be used, which makes the reaction of the absorbent resin and crosslinking agent inefficient. In addition, when a relatively large quantity of the solvent or dispersing medium is used, not only the agglomerate is readily produced, but also the energy cost of the post-treatment process increases undesirably.
Further, the conventional methods can change (improve) the balance of the notable properties of the absorbent resin to a certain extent, but not beyond the extent of practical use. For example, the recent sanitary goods tend to use more amount of absorbent resin while reducing its thickness. However, if the desired properties for the absorbent resin used in the materials of the sanitary goods, that is, an absorbent material, the conventional methods can not balance the properties in a practical manner. Therefore, there has been an increasing need for a method of balancing the properties in a practical manner, in other words, a method of further improving the quality of the absorbent resin.
More specifically, in case of an absorbent material containing a large amount of absorbent resin, namely, having a high concentration of absorbent resin, the desired properties are the absorbing property under pressure, such as absorbency and water retaining ability under pressure, bonding and shape-keeping properties when water is absorbed into spaces among the particles of the absorbent resin which are disclosed in Japanese Laid-open Patent Application No. 96159/1993 (Tokukaihei No. 5-96159), and bonding and shape-keeping properties of an absorbent material made of the absorbent resin, cellulose fiber, etc. when the absorbent material has absorbed water. However, the conventional methods are not effective enough to further improve the absorbing property under pressure. In addition, when the method disclosed in above Japanese Laid-open Patent Application No. 96159/1993 (Tokukaihei No. 5-96159) is adopted, although it becomes possible to control the inconveniences, for example, the absorbent resin is released from the absorbent material or migrates within the absorbent material while water is being absorbed, the absorbency under pressure can be hardly improved in some kinds of absorbent resin. Further, once the absorbent resin is produced, its bonding property deteriorates over time before the absorbent resin is actually used.
Therefore, the conventional methods cause a number of problems specified as above in a reaction of the absorbent resin and crosslinking agent, namely, in a reaction of a hydrophilic polymer and a denaturant.
The present invention is devised to solve the above problems, and therefore, has an object to provide a new denaturing method of a hydrophilic polymer which does not cause the above specified problems. The present invention has another object to provide a new producing method of a hydrophilic resin which does not cause the above specified problems.
The present invention has a further object to provide a producing method of a hydrophilic resin which has an excellent absorbing property under pressure, such as absorbency and water retaining ability under pressure, and can show excellent performance (absorbing property) even when used in the sanitary goods or the like having a high percent by weight of a hydrophilic polymer (high resin concentration).
The present invention has still another object to provide a producing method of a hydrophilic resin which has an excellent absorbing property under pressure, and when used in an absorbent material, a hydrophilic polymer contained therein is hardly released from the absorbent material while water is being absorbed; moreover, the hydrophilic resin maintains its bonding property over time after the production until being put into practical use.
The inventors of the present invention concentrated on the study to achieve the above objects, and discovered that the hydrophilic polymer can be readily and uniformly denatured without causing any of the above problems by being denatured with a denaturant turned into a gaseous state. The inventors of the present invention also discovered that the hydrophilic resin can be readily produced in a simple manner by reacting the hydrophilic polymer with the gaseous denaturant. Further, the inventors of the present invention discovered that the hydrophilic resin can be readily produced in a simple manner by denaturing the hydrophilic polymer with a powdery denaturant substance made from a liquid denaturant substance.
In short, the denaturing method of the hydrophilic polymer of the present invention is characterized in that the hydrophilic polymer is denatured with a gaseous denaturant to fulfil the above objects.
According to a first denaturing method, since the hydrophilic polymer is denatured with a gaseous denaturant, the hydrophilic polymer can be readily, uniformly denatured in a simple manner compared with a case where the hydrophilic polymer and denaturant are reacted with each other in a so-called solid-liquid system. In addition, the solvent or dispersing medium which are indispensable in the conventional methods can be omitted. Since the post-treatment process, such as the solvent or dispersing medium removing step and drying step, can be omitted, the entire denaturation procedure can be simplified and less expensive compared with the conventional methods. Further, since the denaturant and solvent or dispersing medium do not remain in the denatured hydrophilic polymer, the resulting hydrophilic polymer is quite safe. Furthermore, since the hydrophilic polymer is denatured with a gaseous denaturant, not only the hydrophilic polymer and denaturant can react with each other efficiently, but also an excessive denaturant can be readily removed and collected in a simple manner when the denaturation ends. Moreover, the collected denaturant can be readily recycled.
Using a gaseous denaturant can also make it possible to denature the hydrophilic polymer uniformly regardless of its shape and size. Thus, the hydrophilic polymer of a shape which can not be treated by the conventional methods, such as a sheet, film, plate, or block of hydrophilic polymer, or even a porous hydrophilic polymer can be denatured in the present invention. For example, microscopic powders of the hydrophilic polymer can be denatured in the present invention. In short, the denaturing method of the present invention is applicable to any hydrophilic polymer regardless of its shape and size. Further, the hydrophilic polymer is protected from physical damages. For example, the surface of the denatured hydrophilic polymer is not damaged at all.
In case that a crosslinking agent is used as the denaturant, the crosslinking treatment can be also applied to the hydrophilic polymer. In case that the hydrophilic polymer is an absorbent resin, the properties, such as an absorbing property, of the denatured hydrophilic polymer can be improved, To solve the above problems, the producing method of the hydrophilic resin of the present invention is characterized in that the hydrophilic polymer and a gaseous denaturant are reacted with each other.
According to the above arrangement, since the hydrophilic polymer and a gaseous denaturant are reacted with each other, the solvent or dispersing medium which is indispensable in the conventional methods can be omitted. Thus, since the post-treatment process, such as the removing step or drying step, can be omitted, the procedure of the above reaction can be simplified and less expensive compared with the conventional methods. Also, because the denaturant and solvent or dispersing medium do not remain in the resulting hydrophilic resin obtained as a reaction product, the resulting hydrophilic resin is quite safe. Furthermore, since the hydrophilic polymer is denatured with the gaseous denaturant, not only the hydrophilic polymer and denaturant can react with each other efficiently, but also an excessive denaturant can be readily removed and collected in a simple manner when the reaction ends. Moreover, the collected denaturant can be readily recycled. Consequently, it has become possible to produce the hydrophilic resin readily in a simple manner.
To solve the above problems, another producing method of the hydrophilic resin of the present invention is characterized in that the hydrophilic polymer is mixed with a powdery denaturant substance made from a liquid denaturant substance, for example, by mixing the liquid denaturant substance with a water-insoluble compound, cooling the liquid denaturant substance to or below the melting point, or the like.
In the conventional methods, to denature the hydrophilic polymer, more specifically, to apply a surface crosslinking treatment to an absorbent resin using a crosslinking agent, for example, the absorbent resin is mixed with a the crosslinking agent alone or an aqueous solution of the same. Generally, the crosslinking agent is in a liquid state at room temperature and has high affinity with the absorbent resin. For this reason, the absorbent resin starts to absorb the crosslinking agent, or the reaction of the absorbent resin and crosslinking agent takes place as soon as the absorbent resin and crosslinking agent are mixed with each other. Thus, the crosslinking agent can not be distributed evenly on the surface of the absorbent resin, and hence, the surface crosslinking treatment can not be applied uniformly to the absorbent resin.
In contrast, according to the producing method of the hydrophilic resin of the present invention, a liquid denaturant substance is made into powders first, and thence mixed with the hydrophilic polymer. Thus, the hydrophilic polymer can be mixed with the liquid denaturant substance which is substantially turned into a solid state. Consequently, compared with a case where the hydrophilic polymer is mixed with the liquid denaturant substance directly, the hydrophilic polymer can be mixed homogeneously with the liquid denaturant substance which has been made into powders in effect. Also, according to the producing method of the hydrophilic resin of the present invention, the liquid denaturant substance is made into powders. Thus, when the hydrophilic polymer and powdery denaturant substance are mixed with each other, the hydrophilic polymer does not start to absorb the liquid denaturant substance contained in the powdery denaturant substance, nor does the reaction of the hydrophilic polymer and the denaturant contained in the liquid denaturant substance take place immediately. The hydrophilic polymer is denatured when a mixture of the hydrophilic polymer and powdery denaturant substance has absorbed an aqueous liquid or when the mixture is heated. Therefore, according to the producing method of the hydrophilic resin of the present invention, since the hydrophilic polymer and powdery denaturant substance can be mixed with each other homogeneously, not only the hydrophilic polymer can be denatured uniformly, but also the denaturation timing can be controlled.
Thus, according to the above method, not only the hydrophilic polymer can be denatured uniformly, but also the hydrophilic resin with a good balance of the properties can be obtained.
In the following, the present invention will be described in detail.
In the present invention, a gaseous denaturant or a powdery denaturant substance made from a liquid denaturant substance are used to denature the hydrophilic polymer uniformly. Note that, in the present invention, xe2x80x9cdenaturationxe2x80x9d means to change the structure or physical properties of the hydrophilic polymer by bringing the hydrophilic polymer into contact with the denaturant or reacting the hydrophilic polymer with the denaturant. To be more specific, xe2x80x9cdenaturationxe2x80x9d means, for example, a crosslinking treatment (secondary crosslinking treatment). The reaction of the hydrophilic polymer and denaturant includes a crosslinking reaction, an addition reaction, a substitute reaction, an esterification reaction, etc.
The hydrophilic polymer subject to denaturation (hereinafter, referred to as treatment) by the denaturing method and producing method of the present invention is not especially limited, and a solid or gel of any hydrophilic polymer having a reaction group (functional group) is applicable. A carboxyl group is particularly preferred as the above reaction group. Also, the hydrophilic polymer having a crosslinking structure inside is preferred. More specifically, a good example of the hydrophilic polymer is partially neutralized poly(meth)acrylic acid having the crosslinking structure inside, such as an absorbent resin.
The hydrophilic polymer can be obtained by, for example, polymerizing a monomeric composition mainly composed of acrylic acid and a salt thereof (hereinafter, referred to as acrylic acid/salt). Examples of the hydrophilic polymer are known absorbent resins including: a partially neutralized crosslinked polymer of polyacrylic acid (U.S. Pat. Nos. 4,625,001, 4,654,039, 5,250,640, and 5,275,773, and European Patent No. 456,136), a hydrolyzed graft polymer of starch-acrylonitrile, a neutralized or partially neutralized crosslinked graft polymer of starch-acrylic acid (U.S. Pat. No. 4,076,663), a saponified copolymer of vinyl acetate-acrylic acid (U.S. Pat. No. 4,124,748), a saponified copolymer of vinyl acetate-acrylic ester, a hydrolyzed (co)polymer of acrylonitrile (U.S. Pat. No. 3,935,099) or a crosslinked product thereof, a hydrolyzed copolymer of acrylamide (U.S. Pat. No. 3,959,569) or a crosslinked product thereof, a crosslinked carboxymethyl cellulose, a crosslinked cationic monomer, a crosslinked copolymer of isobutylene-maleic anhydride, a crosslinked copolymer of 2-acrylamide-2-methylpropane sulfonic acid and acrylic acid, a crosslinked polyethylene oxide, a crosslinked copolymer of methoxypolyethylene glycol and acrylic acid, etc. Of all the above example hydrophilic polymers, crosslinked polyacrylic acid/salt is preferred. It is preferable that 50 mol%-90 mol% of acidic groups in the crosslinked polyacrylic acid/salt are neutralized. Also, alkali metal salt, alkali earth metal salt, ammonium salt, hydroxyammonium salt, amine salt, alkylamine salt, etc. are preferred as the salt.
Besides acrylic acid/salt, the monomeric composition may optionally contain a hydrophilic monomer copolymerizable with acrylic acid/salt. Examples of the hydrophilic monomer include:
anionic unsaturated monomers, such as methacrylic acid, crotonic acid, maleic acid, maleic anhydride, fumaric acid, itaconic acid, cinnamic acid, sorbic acid, xcex2-acryloyloxypropionic acid, 2-(meth)acryloylethane sulfonic acid, 2-(meta)acryloylpropane sulfonic acid, 2-(meth)acrylamide-2-methylpropane sulfonic acid, vinyl sulfonic acid, styrene sulfonic acid, allyl sulfonic acid, vinyl phosponic acid, and 2-(meth)acryloyloxyethyl phosphoric acid, or salts of these acids (for example, alkali metal salt, alkali earth metal salt, ammonium salt, and alkylamine salt);
nonionic unsaturated monomers, such as acrylamide, methacrylamide, N-ethyl(meth)acrylamide, N-n-propyl(meth)acrylamide, N-isopropyl(meth)acrylamide, N,N-dimethyl(meth)acrylamide, 2-hydroxyethyl(meth)acrylate, 2-hydroxypropyl(meth)acrylate, methoxypolyethylene glycol (meth)acrylate, polyethylene glycol mono(meth)acrylate, vinylpyridine, N-vinylpyrrolidone, N-acryloylpiperidine, and N-acryloylpyrrolidine;
cationic unsaturated monomers, such as N,N-dimethylaminoethyl(meth)acrylate, N,N-diethylaminoethyl(meth)acrylate, N,N-dimethylaminopropyl(meth)acrylate, and N,N-dimethylaminopropyl(meth)acrylamide, or quaternary compounds thereof (for example, a reaction product with alkyl halide, or a reaction product with dialkyl sulfric acid); etc.
One member or a mixture of more than one member selected from these example hydrophilic monomers can be used effectively.
In case that the monomeric composition is mainly composed of acrylic acid/salt, an amount of the hydrophilic monomers other than acrylic acid/salt is preferably below 50 mol%, more preferably 30 mol% or below, and most preferably 10 mol% or below based on the combined weight of the monomeric compositions.
It is preferable that the hydrophilic polymer produced by (co)polymerizing the above monomeric composition(s) has(have) the crosslinking structure (primary crosslinking structure) inside. The above crosslinking structure can be readily introduced into the hydrophilic polymer using an internal crosslinking agent when the monomeric composition is copolymerized, so that the monomeric composition copolymerizes or reacts with the internal crosslinking agent. The hydrophilic polymer may be of a self-crosslinking type that does not need the internal crosslinking agent.
Examples of the internal crosslinking agent include: a compound having a plurality of vinyl groups (polymeric unsaturated groups) within a molecule; a compound having at least one vinyl group and at least one functional group reactive with a reaction group contained in the monomeric composition within a molecule; a compound having a plurality of functional groups reactive with the above reaction group within a molecule; etc. One member or a mixture of more than one member selected from these example internal crosslinking agents can be used effectively.
Examples of the compound having a plurality of vinyl groups within a molecule include N,Nxe2x80x2-methylenebis(meth)acrylamide, (poly)ethylene glycol di(meth)acrylate, (poly)propylene glycol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, trimethylolpropane di(meth)acrylate, glycerin tri(meth)acrylate, glycerin acrylate methacrylate, ethyleneoxide denatured trimethylolpropane tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol hexa(meth)acrylate, N,N-diallylacrylamide, triallyl cyanurate, triallyl isocyanurate, triallyl phosphate, triallyl amine, diallyloxy acetic acid, N-methyl-N-vinylacrylamide, bis (N-vinyl carboxylic amide), poly(meth)allyloxyalkanes, such as tetraallyloxy ethane, etc.
The compound having at least one vinyl group and at least one functional group reactive with the reaction group within a molecule means an ethylenic unsaturated compound having at least one of a hydroxyl group, an epoxy group, a cationic group and the like, and the examples of which include: glycidyl(meth)acrylate, N-methylol acrylamide, (meth)acrylic acid dimethylamino ethyl, etc.
The compound having a plurality of functional groups reactive with the reaction group within a molecule means, for example, a compound having at least two groups selected from a hydroxyl group, an epoxy group, a cationic group, an isocyanate group, etc. Examples of such a compound include: (poly)ethylene glycol diglycidyl ether, glycerol diglycidyl ether, ethylene glycol, polyethylene glycol, propylene glycol, glycerin, pentaerythritol, ethylene diamine, ethylene carbonate, polyethyleneimine, aluminum sulfate, etc.
Of all these example internal crosslinking agents, the compound having a plurality of vinyl groups within a molecule is preferred, because using such a compound can further improve the properties of the resulting hydrophilic polymer, for example, the absorbing property in case that the hydrophilic polymer is an absorbent resin. Although an amount of the internal crosslinking agent with respect to an amount of the hydrophilic monomer varies with a combination of the hydrophilic monomer and internal crosslinking agent, a preferable range is between 0.005 mol% and 3 mol%, and a more preferable range is between 0.01 mol% and 1.5 mol%. If less than 0.005 mol% or more than 3 mol% of the internal crosslinking agent is used when the hydrophilic polymer is the absorbent resin, for example, the resulting absorbent resin may not have the desired absorbing property.
When the monomeric composition is (co)polymerized, hydrophilic high polymers, such as starch or derivatives of starch, cellulose or derivatives of cellulose, polyvinylalcohol, polyacrylic acid/salt and a crosslinked product of the same, or chain transfer agents, such as hypophosphorous acid and hypophosphite, may be added to the reactant system.
The polymerization method for (co)polymerizing the monomeric composition is not especially limited, and known methods, such as the aqueous solution polymerization, reverse phase suspension polymerization, bulk polymerization, and precipitation polymerization, can be adopted. Of these polymerization methods, the methods in which an aqueous solution of the monomeric composition is polymerized, that is, the aqueous solution polymerization and reverse phase suspension polymerization are preferable because the polymerization reaction can be readily controlled and the properties of the resulting hydrophilic polymer can be further improved. The reaction conditions, such as reaction temperature and time, are not especially limited, and can be set arbitrarily depending on the kinds of the hydrophilic monomer or the like. The aqueous solution polymerization and reverse phase suspension polymerization can be carried out by known polymerization methods disclosed in, for example, U.S. Pat. Nos. 4,625,001, 4,769,427, 4,873,299, 4,093,776, 4,367,323, 4,446,261, 4,683,274, 4,690,996, 4,721,647, 4,738,867, and 4,748,076.
When the monomeric composition is (co)polymerized, a radical polymerization initiator or activation energy rays, such as UV rays and electron beams, can be used. Examples of the radical polymerization initiator include peroxides, such as potassium persulfate, sodium persulfate, ammonium persulfate, t-butylhydro peroxide, hydrogen peroxide, 2,2xe2x80x2-azobis (2-amidinopropane) dihydrochloride, 2,2xe2x80x2-azobisisobutyronitrile benzoyl peroxide, cumene hydroperoxide, and di-t-butylperoxide. One member or a mixture of more than one member selected from these radical polymerization initiators can be used effectively. An oxidizing radical polymerization initiator may be used as a redox initiator when combined with a reducing agent, such as sulfites including sodium sulfite and sodium hydrogen sulfite, bisulfite, thiosulfate, formamidine sulfenic acid, ferrous sulfate, and L-ascorbic acid. An amount of the polymerization initiator with respect to an amount of the hydrophilic monomer varies with a combination of the hydrophilic monomer and polymerization initiator or the like, but a preferable range is between 0.001 mol% and 2 mol%, and a more preferable range is between 0.01 mol% and 0.5 mol%.
The shape of the hydrophilic polymer obtained as the result of the above (co)polymerization is not especially limited, and the hydrophilic polymer can be spherical, substantially spherical, granular, leaflet, flat, etc. particles; a fiber; a bar, a sheet, a film, a plate, a block, or have an undefined shape (pulverized undefinedly); etc. The hydrophilic polymer may be a porous product or a sponge having sequential pores therein. The size of the hydrophilic polymer is not especially limited either, and the hydrophilic polymer may be microscopic powders. In other words, the hydrophilic polymer does not have to be of specific shape and size when treated by the denaturant, and can be of any suitable shape and size for the intended use. In short, the denaturing method of the present invention is applicable to any hydrophilic polymer regardless of its shape and size.
The particle size of the hydrophilic polymer may be adjusted through sieving or the like when necessary. In case that the hydrophilic polymer is an absorbent resin, a preferable particle size is in a range between 200 xcexcm and 600 xcexcm. In this case, it is further preferable that the particles having a particle size of smaller than 150 xcexcm are 10 wt% or less, and more preferably 5 wt% or less, based on the weight of the hydrophilic polymer. When an average particle size of the hydrophilic polymer is outside of the above range, it becomes difficult to obtain the hydrophilic polymer with an excellent absorbing property.
According to the denaturing method of the present invention, the hydrophilic polymer is denatured with, for example, a gaseous denaturant. In practice, the hydrophilic polymer is brought into contact with the gaseous denaturant to be denatured. According to the producing method of the present invention, the hydrophilic polymer and gaseous denaturant are reacted with each other to obtain the hydrophilic resin as a reaction product.
In this case, the denaturant only has to be in the gaseous state (vapor state) when brought into contact with the hydrophilic polymer. In other words, the denaturant used in the above denaturing and producing methods is not especially limited, and any compound that turns into a gas under gasfication conditions at or above the boiling point and reacts with a reaction group contained in the hydrophilic polymer in the gaseous state, that is, a compound reactive in the so-called solid-vapor system, is applicable. Alternatively, the denaturant may be a compound that turns into a gas under gasfication conditions at or above the boiling point, and turns into a liquid upon contact with the hydrophilic polymer while reacting with the functional group (reaction group) contained in the hydrophilic polymer in the liquid state. Further, the denaturant may be a compound that turns into a solid upon contact with the hydrophilic polymer and reacts with the functional group (reaction group) contained in the hydrophilic polymer in the solid state. In short, the denaturant only has to be in the gaseous state when brought into contact with the hydrophilic polymer, and the denaturant can be of any state (gas, liquid or solid) when reacting with the reaction group contained in the hydrophilic polymer.
A so-called crosslinking agent is preferable as the denaturant, and examples of which include:
alkylene oxide compounds, such as ethylene oxide (boiling point: 10.7xc2x0 C./760 mmHg) and propylene oxide (boiling point: 34.2xc2x0 C./760 mmHg);
alkyleneimine compounds, such as ethyleneimine (boiling point: 56xc2x0 C./760 mmHg), propyleneimine (boiling point: 67xc2x0 C./760 mmHg);
polyglycidyl ether compounds, such as ethylene glycol diglycidyl ether (boiling point: 125xc2x0 C./5 mmHg), neopentyl glycol diglycidyl ether (boiling point: 125xc2x0 C./1 mmHg), and glycerol triglycidyl ether (boiling point: 195xc2x0 C./1.5 mmHg);
alkylene carbonate compounds, such as ethylene carbonate (boiling point: 100xc2x0 C./5 mmHg) and propylene carbonate (boiling point: 242xc2x0 C./760 mmHg);
polyhydric alcohol compounds, such as ethylene glycol (boiling point: 70xc2x0 C./3 mmHg), diethylene glycol (boiling point: 244xc2x0 C./760 mmHg), triethylene glycol (boiling point: 287xc2x0 C./760 mmHg), and glycerin (boiling point: 290xc2x0 C./760 mmHg);
polyamine compounds, such as ethylene diamine (boiling point: 116xc2x0 C./760 mmHg), hexamethylene diamine (boiling point: 196xc2x0 C./760 mmHg), diethylene triamine (boiling point: 207xc2x0 C./760 mmHg), triethylene tetramine (boiling point: 287xc2x0 C./760 mmHg), and tetramethyl ethylene diamine (boiling point: 120xc2x0 C./760 mmHg);
haloepoxy compounds, such as epichlorohydrin (boiling point: 62xc2x0 C./100 mmHg);
polyaldehyde compounds, such as glutaraldehyde (boiling point: 72xc2x0 C./10 mmHg) and glyoxal (boiling point: 51xc2x0 C./776 mmHg);
alkylene sulfide compounds, such as ethylene sulfide (boiling point: 53xc2x0 C./760 mmHg) and propylene sulfide (boiling point: 70xc2x0 C./760 mmHg); etc.
One member or a mixture of more than one member selected from these denaturants can be used effectively. Using a mixture of more than one denaturant can denature the hydrophilic polymer in several manners concurrently, namely, in a single step.
To turn the above example compounds into a gas, a vapor pressure is raised above the pressure inside the denaturation system (hereinafter, referred to as treatment system). More specifically, the denaturant is turned into a gas either by heating the treatment system inside at or above the boiling point of the denaturant being used, or reducing a pressure inside the treatment system below a vapor pressure of the denaturant being used, or a combination of both.
To be more specific, in case of ethylene oxide, as can be understood from Table 1 below showing a temperature-vapor pressure relation, under the pressuring condition at about 5 Kgf/cm2, for example, ethylene oxide is tuned into a gas at or above 50xc2x0 C. In this manner, the gasfication conditions can be changed arbitrary for the compound being used as the denaturant, and as a consequence, a so-called solid-vapor reaction can be realized.
The treatment conditions under which the hydrophilic polymer is treated with a gaseous denaturant are not especially limited as long as the denaturant remains in the gaseous state. For example, the treatment is carried out under a reduced, normal (ambient), or applying pressure. To treat the hydrophilic polymer at a high degree, that is, to obtain relatively high crosslinking density or crosslinking depth, there may be a case that the treatment under a normal pressure is more preferable than the treatment under a reduced pressure, and also there may be a case that the treatment under an applying pressure is more preferable than the treatment under a normal pressure. In short, the treatment pressure can be set arbitrarily depending on the desired treatment degree. Manipulating the treatment pressure makes it possible to readily control the treatment degree of the hydrophilic polymer.
Although a treatment temperature varies with the treatment pressure or the reactivity of the hydrophilic polymer and denaturant, the treatment temperature is preferably in a range between room temperature and 300xc2x0 C., more preferably in a range between 100xc2x0 C. and 250xc2x0 C., and most preferably in a range between 130xc2x0 C. and 230xc2x0 C. A treatment time is not especially limited and can be set arbitrarily in response to the treatment temperature and pressure, or reactivity of the hydrophilic polymer and denaturant. The treatment time is preferably in a range between a few seconds and 2 hours, and more preferably in a range between a few minutes and 1 hour. Note that the reaction of the hydrophilic polymer and denaturant can take place in a reactant system with no water, that is, an anhydrous state. In other words, the denaturing method of the present invention is not affected whether there is water in the treatment system or not.
A treatment apparatus is not especially limited and only has to include an arrangement such that the hydrophilic polymer (hereinafter, simply referred to as polymer) can be brought into contact with a gaseous denaturant (hereinafter, sometimes simply referred to as gas in a satisfactory manner, that is, an arrangement such that the solid-vapor system reaction can be realized. Known reacting apparatuses can be used as the treatment apparatus. For example, {circle around (1)} a moving bed type reacting apparatus for moving the polymer gradually to trigger the reaction with the gas upon contact; {circle around (2)} a fluidized bed type reacting apparatus for keeping the polymer afloat and suspended in the gas to trigger the reaction upon contact; {circle around (3)} a fixed bed type reacting apparatus for keeping the polymer as a fixed bed while moving the gas in a single phase current, counter current, or parallel current to trigger the reaction upon contact; {circle around (4)} stirring bath type reacting apparatus for stirring the polymer and gas in the bath using a stirring blade to trigger the reaction upon contact; {circle around (5)} a flash type reacting apparatus for blowing out the polymer with a gas flow to trigger the reaction upon contact; etc.
Examples of the moving bed type reacting apparatus are illustrated in FIGS. 1 through 7. More specifically, examples include: a counter-current stand type reacting apparatus of FIGS. 1 or 2, in which the polymer moves downwards while the gas moves upwards; a cross type reacting apparatus of FIG. 3, in which the polymer moves downward while the gas moves transversely; migration grate type reacting apparatus of FIGS. 4 or 5, in which the polymer is conveyed horizontally by a belt conveyor while the gas moves upwards; a rotary kiln type reacting apparatus of FIG. 6, in which the polymer and gas are moved in the same direction as the apparatus rotates; a multi-stage kiln type reacting apparatus of FIG. 7, in which a number of stages are made inside the apparatus, so that the polymer moves downward step by step while the gas moves upward; etc. Note that the migration grate type reacting apparatus of FIG. 5 is particularly suitable when a sheet of polymer is denatured.
Examples of the moving bed type reacting apparatus are illustrated in FIGS. 8 through 11. More specifically, examples include: a vapor-solid moving bed type reacting apparatus of FIGS. 8 or 9, in which the polymer is afloat or suspended in the apparatus furnished with inner members, such as a porous plate, a metal gauze, and a pipe, while the gas moves upwards; a high-speed moving bed type reacting apparatus of FIG. 10, in which the polymer is afloat or suspended by moving the gas upward at a high speed; a jet bed type reacting apparatus of FIG. 11, in which the polymer is afloat or suspended by an upward jet of the gas; etc.
Examples of the stirring bath type reacting apparatus are illustrated in FIGS. 12 and 13. More specifically, examples include: a vapor-solid stirring bath type reacting apparatus of FIG. 12, in which the polymer and gas in the bath are stirred by the stirring blade; and a multi-stage blade bath type reacting apparatus of FIG. 13, in which a number of partitions are provided in the bath, so that the polymer and gas are moving upward step by step while being stirred by the stirring blade; etc.
Also, an example of the flash type reacting apparatus is illustrated in FIG. 14, which is a vapor-solid flash type reacting apparatus for blowing out the polymer with the gas flow.
The structure of the treatment apparatus, that is, the reacting apparatus, is not limited to those of the above examples. The treatment apparatus can be either the batch type or continuous type. For example, a reacting apparatus of a sealed system, such as an autoclave, can be suitably used as the treatment apparatus. In short, the denaturing method of the present invention can be carried out satisfactorily by the treatment apparatus of either the batch type or continuous type. Also, the hydrophilic polymer can be treated uniformly by bringing the hydrophilic polymer into contact with a gaseous denaturant in a satisfactory manner by the treatment apparatus under predetermined conditions. As previously mentioned, since the denaturing method of the present invention uses a gas, the denaturation takes a relatively short time and completes in a reliable manner.
As has been explained, the denaturing method of the hydrophilic polymer of the present invention is the method of denaturing the hydrophilic polymer with a gaseous denaturant. The hydrophilic polymer and denaturant are, for example, an absorbent resin and a crosslinking agent, respectively.
According to the above method, the solvent or dispersing medium which is indispensable in the conventional methods can be omitted. Thus, the denaturation procedure can be simplified and less expensive compared with the conventional methods. Also, since the denaturant and solvent or dispersing medium do not remain in the denatured hydrophilic polymer, the denatured hydrophilic polymer is quite safe. Further, not only can the hydrophilic polymer and denaturant react with each other efficiently, but also an excessive denaturant can be readily removed and collected in a simple manner when the denaturation ends. Moreover, the collected denaturant can be readily recycled.
Additionally, according to the above method, the hydrophilic polymer can be denatured uniformly regardless of its size and shape. Thus, the hydrophilic polymer of some specific shapes, or porous hydrophilic polymer which can not be treated by the conventional methods can be denatured by the above method. Further, for example, even microscopic powders of the hydrophilic polymer can be denatured by the above method. In short, the above method can be applied to any hydrophilic polymer regardless of its shape and size. Furthermore, a physical damage, such as the damage to the surface of the denatured hydrophilic polymer, can be prevented.
In case a crosslinking agent is used as the gaseous denaturant, the crosslinking treatment can be also applied to the hydrophilic polymer. Also, in case that the hydrophilic polymer is an absorbent resin, the properties, such as an absorbing property, can be improved.
As has been explained, the producing method of the hydrophilic resin of the present invention is a method of reacting the hydrophilic polymer with a gaseous denaturant. According to the above method, since the hydrophilic polymer and gaseous denaturant are reacted with each other, the solvent or dispersing medium which is indispensable in the conventional methods can be omitted. Thus, since the post-treatment procedure, such as the removing step or drying step, can be omitted, the reaction procedure can be simplified and less expensive compared with the conventional methods. Also, since the denaturant and solvent or dispersing medium do not remain in the hydrophilic resin obtained as a reaction product, the resulting hydrophilic resin is quite safe. Further, since the hydrophilic polymer is reacted with the gaseous denaturant, not only the reaction takes place efficiently, but also an excessive denaturant can be readily removed and collected in a simple manner when the reaction ends. Furthermore, the collected denaturant can be readily recycled. Consequently, it has become possible to produce the hydrophilic resin readily in a simple manner.
Also, another producing method of the hydrophilic resin of the present invention is a method of mixing the hydrophilic polymer with a powdery denaturant substance made from a liquid denaturant substance. In the present invention, the liquid denaturant substance means a denaturant substance that remains in the liquid state when being added to the hydrophilic polymer. The liquid denaturant substance can be a pure liquid of the denaturant or a solution or dispersing liquid prepared by dissolving or dispersing the denaturant into an adequate solvent. In other words, the liquid denaturant substance is not especially limited as long as it contains the denaturant and remains in the liquid state when being added to the hydrophilic polymer.
The denaturant is not especially limited either, and can be any compound reactive with a reaction group contained in the hydrophilic polymer. In this case, the denaturant is a compound having a number of functional groups reactive with a reaction group contained in the hydrophilic polymer within a molecule (that is, multifunctional compound), or a compound having a single functional group reactive with a reaction group contained in the hydrophilic polymer within a molecule (unifunctional compound).
An example of the compound having a number of functional groups reactive with a reaction group contained in the hydrophilic polymer within a molecule and serving as the denaturant is a crosslinking agent (surface crosslinking agent). In case that the reaction group contained in the hydrophilic polymer is a carboxyl group, examples of preferred crosslinking agent include, but are not limited to the known surface crosslinking agents as follow:
polyhydric alcohols, such as ethylene glycol, propylene glycol, diethylene glycol, dipropylene glycol, triethylene glycol, tripropylene glycol, tetraethylene glycol, tetrapropylene glycol, polyethylene glycol, polypropylene glycol, 1,3-propanediol, 2,2,4-trimethyl-1,3-pentanediol, glycerin, polyglycerin, 2-butene-1,4-diol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 2,4-pentanediol, 1,6-hexanediol, 2,5-hexanediol, 1,2-cyclohexane dimethanol, 1,2-cyclohexanol, trimethylolpropane, diethanolamine, triethanolamine, polyoxypropylene, a block copolymer of oxyethylene-oxypropylene, pentaerythritol, and sorbitol;
polyepoxides, such as ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, glycerol polyglycidyl ether, diglycerol polyglycidyl ether, polyglycerol polyglycidyl ether, propylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, and glycidol;
polyamines, such as ethylene diamine, diethylene triamine, triethylene tetramine, tetraethylene pentamine, pentaethylene hexamine, polyallylamine, and polyethyleneimine;
alkylene carbonates, such as 1,3-dioxolane-2-one, 4-methyl-1,3-dioxolane-2-one, 4,5-dimethyl-1,3-dioxolane-2-one, 4,4-dimethyl-1,3-dioxolane-2-one, 4-ethyl-1,3-dioxolane-2-one, 4-hydroxymethyl-1,3-dioxolane-2-one, 1,3-dioxane-2-one, 4-methyl-1,3-dioxane-2-one, and 4,6-dimethyl-1,3-dioxane-2-one;
polyisocyanates, such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate;
polyoxazoline compounds, such as 1,2-ethylenebis oxazoline;
haloepoxides, such as epichlorohydrin, epibromohydrin, xcex1-methylepichlorohydrin;
polyvalent metal compounds, namely, a hydroxide and a chloride of polyvalent metals, such as zinc, calcium, magnesium, aluminum, iron, and zirconium; etc.
One member or a mixture of more than one member selected from these crosslinking agents can be used effectively.
Of all these examples, a crosslinking agent containing at least one kind of compound selected from a group consisting of polyhydric alcohol, polyepoxide, polyamine, and alkylene carbonate is preferred.
Of all the polyamines, a polyamine (hereinafter, referred to as high molecular weight polyamine) having a weight average molecular weight (Mw) of 2,000 or greater is preferable, and a high molecular weight polyamine having a weight average molecular weight ranging from 10,000 to 10,000,000 is particularly preferable. If a high molecular weight polyamine having a weight average molecular weight less than 2,000 is used, the resulting hydrophilic resin may not have satisfactory bonding property or shape-keeping property when water is being absorbed into spaces among particles of the hydrophilic polymer, or an absorbent material made of the hydrophilic resin and cellulose fiber (for example, paper and comminuted pulp) may not have satisfactory bonding property or shape-keeping property when water is being absorbed.
In case that the crosslinking agent includes the high molecular weight polyamine, the hydrophilic polymer and high molecular weight polyamine contact and start to react with each other when the hydrophilic resin, namely, the hydrophilic polymer contained in the hydrophilic resin, has absorbed an aqueous liquid, whereupon the bonding force among the particles of the hydrophilic polymer or shape-keeping ability of the absorbent product is developed. Therefore, according to the present invention, the reaction timing of the hydrophilic polymer and high molecular weight polyamine can be controlled, and the deterioration of the bonding property over time can be prevented more effectively. In case that the high molecular weight polyamine is used as the crosslinking agent, the surface crosslinking treatment may be applied to the surface portion of the hydrophilic polymer in advance by any known surface crosslinking method or the method of the present invention.
Examples of the high molecular weight polyamine include:
(1) a monopolymer of monoallylamine derivative and a monopolymer of a diallylamine derivative;
(2) a copolymer of more than one kind of monoallylamine derivatives, a copolymer of more than one kind of diallylamine derivatives, a copolymer of a monoallylamine derivative and a diallylamine derivative;
(3) a copolymer of a monoallylamine derivative and/or diallylamine derivative, and a dialkyldiallylammonium salt;
(4) a homopolymer of an unsaturated carboxylic acid derivative containing a tertiary amino group (hereinafter referred to as the unsaturated carboxylic acid derivative a);
(5) a copolymer of more than one kind of the unsaturated carboxylic acid derivatives a;
(6) a copolymer of the unsaturated carboxylic acid derivative a, and a protonic and/or alkylated tertiary amino group of the unsaturated carboxylic acid derivative a (hereinafter simply referred to as quaternary ammonium salt), and/or a dialkyldiallylammonium salt;
(7) a ternary copolymer of the unsaturated carboxylic acid derivatives a, quaternary ammonium salt and/or a dialkyldiallylammonium salt, and a vinyl monomer copolymerizable with the above monomers;
(8) a polymer produced by copolymerizing an unsaturated carboxylic acid and an unsaturated monomer copolymerizable with the unsaturated carboxylic acid first, and thence reacting alkyleneimine with a carboxyl group contained in the resulting copolymer;
(9) polyalkyleneimine;
(10) polyalkyleneimine-epihalohydrin resin;
(11) polyalkylenepolyamine;
(12) a polymer of (2-methacryloyloxyethyl) ethyleneimine, and a copolymer of (2-methacryloyloxyethyl)ethyleneimine and an unsaturated monomer copolymerizable with (2-methacryloyloxyethyl) ethyleneimine;
(13) polyamidepolyamine;
(14) polyamideamine epihalohydrin resin;
(15) denatured polyacrylamide as a result of Mannich reaction and denatured polymethacrylamide as a result of Mannich reaction;
(16) polyvinylamine, and a copolymer of vinylamine and an unsaturated monomer copolymerizable with vinylamine;
(17) a condensation polymer of dicyandiamide-diethylenetriamine; etc.
To be more specific, examples of the high molecular weight polyamine include: polyallylamine, polydiallylamine, poly(N-alkylallylamine), poly(alkyldiallylamine), a copolymer of monoallylamine-diallylamine, a copolymer of N-alkylallylamine-monoallylamine, a copolymer of monoallylamine-dialkyldiallylammonium salt, a copolymer of diallylamine-dialkyldiallylammonium salt, polydimethylaminoethyl acrylate, polydiethylaminoethyl acrylate, polydimethylaminoethyl acrylamide, straight-chain polyethyleneimine, branched-chain polyethyleneimine, polyethylenepolyamine, polypropylenepolyamine, polyamidepolyamine, polyetherpolyamine, polyvinylamine, polyamidepolyamine-epichlorohydrin resin, polyamidine, etc. The examples also include amino denatured products produced by reacting formaldehyde and diethylamine with polyacrylamide or polymethacrylamide. Also, the high molecular weight polyamine may be neutralized by an acidic compound either completely or partially.
Examples of the compound having a single functional group reactive with a reaction group contained in the hydrophilic polymer within a molecule and serving as the denaturant include, but are not limited to:
compounds having a hydroxyl group, such as pentanol, hexanol, heptanol, octanol, decanol, alkoxy polyethylene glycol, lactic acid, and ethyl lactate;
compounds having an epoxy group, such as 2-ethylhexylglycidyl ether, phenylglycidyl ether, butylglycidyl ether, 2-methyloctylglycidyl ether, allylglycidyl ether, and glycidyl(meth)acrylate;
compounds having an amino group, such as methylamine, ethylamine, diethylamine, triethylamine, n-propylamine, isopropylamine, diisopropylamine, 3-methoxy propylamine, 3-ethoxy propylamine, 3-(2-ethylhexyloxy)propylamine, 3- (dibutylamino)propylamine, n-butylamine, t-butylamine, sec-butylamine, diisobutylamine, 2-ethylhexylamine, and di-2-ethylhexylamine, tri-n-octylamine, and salts of these compounds;
cationic surfactants having an amino group and salts of such surfactants; etc.
One member or a mixture of more than one member selected from these compounds can be used effectively. Further, a mixture of the compound having a single functional group within a molecule and the compound having a number of functional groups within a molecule may be used as the denaturant of the present invention.
Although an amount of the denaturant with respect to an amount of the hydrophilic polymer varies with the kinds or combination of the hydrophilic polymer and denaturant or the use of the resulting hydrophilic resin, a preferable amount is in a range between 0.001 part by weight and 10 parts by weight with respect to 100 parts by weight of the hydrophilic polymer in solid, and a more preferable amount is in a range between 0.01 part by weight and 5 parts by weight. Limiting an amount of the denaturant within the above range makes it possible to obtain the hydrophilic resin serving as an absorbing agent with an excellent absorbing property under pressure and bonding and shape-keeping properties. If more than 10 parts by weight of the denaturant are used, a part of the denaturant is wasted uneconomically. Also, an excessive denaturant may prevent to realize a desired denaturing (improving) effect. More specifically, in case that the hydrophilic polymer is an absorbent resin and the denaturant is a crosslinking agent, if the crosslinking agent is used excessively, the crosslinking density becomes too high to maintain good absorbency of the resulting absorbing agent, namely, hydrophilic resin. On the other hand, in case less than 0.001 part by weight of the denaturant is used, the desired denaturing effect can be hardly attained.
In case that the liquid denaturant substance of the present invention contains a solvent, examples of the solvent are water, a hydrophilic organic solvent, and a mixture of water and the hydrophilic organic solvent. In other words, in case that the liquid denaturant substance is a solution or dispersing liquid of the denaturant, the liquid denaturant substance can be readily prepared by dissolving or dispersing the denaturant into, for example, water and/or a hydrophilic organic solvent. A concentration of the denaturant in the solution or dispersing liquid is not especially limited.
Examples of the hydrophilic organic solvent include:
lower alcohols, such as methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, isobutyl alcohol, and t-butyl alcohol;
ketones, such as acetone;
ethers, such as dioxane, tetrahydrofuran;
amides, such as N,N-dimethylformamide;
sulfoxides, such as dimethyl sulfoxide; etc.
A method of producing a powdery denaturant substance by turning the liquid denaturant substance into powders is not especially limited in the present invention. For example, a method of mixing the liquid denaturant substance with powders of a water-insoluble compound, a method of cooling the liquid denaturant substance to or below the melting point, etc. are applicable.
The powders of the water-insoluble compound are not especially limited as long as the water-insoluble compound remains inactive in response to the reaction of the hydrophilic polymer and denaturant and does not affect the properties of the resulting hydrophilic resin.
Examples of the water-insoluble compound include, but are not limited to:
inorganic powders, such as silicon dioxide, titanium dioxide, aluminum oxide, magnesium oxide, zinc oxide, talc, calcium phosphate, barium phosphate, silicic acid, silicate, clay, diatomaceous earth, zeolite, bentonite, kaolin, hydrotalcite, perlite, isolite, activated clay, silica sand, quartzite, strontium ore, fluorite, and bauxite;
organic powders, such as polyethylene, polypropylene, polyvinyl chloride, polystyrene, polyamide, melamine resin, polymethyl methacrylate, denatured starch, powders of cellulose, ethyl cellulose, sawdust, activated carbon, and tea-leaves;
microscopic powders of absorbent resins; etc.
One member or a mixture of more than one member selected from these water-insoluble compounds can be used effectively. Of all these example water-insoluble compounds, microscopic inorganic powders are preferable, and microscopic powders of clay are more preferable. Also, a preferable particle size of the water-insoluble compound is 1,000 xcexcm or less, more preferably 500 xcexcm or less, and most preferably 100 xcexcm or less.
Although a ratio of the liquid denaturant substance and water-insoluble compound in the powdery denaturant substance varies with the kinds and combination of the denaturant and water-insoluble compound or the use of the resulting hydrophilic resin, a ratio of the liquid denaturant substance (that is, denaturant or a solution or dispersing liquid of the same) with respect to 100 parts by weight of the water-insoluble compound is preferably in a range between 1 part by weight and 1,000 parts by weight, and more preferably in a range between 10 parts by weight and 500 parts by weight. When more than 1,000 parts by weight of the liquid denaturant substance is used, the resulting powdery denaturant substance readily turn into a slurry, and when this happens, the hydrophilic polymer starts to absorb or starts to react with the denaturant contained in the powdery denaturant substance as soon as the hydrophilic polymer and powdery denaturant substance are mixed with each other. Consequently, it has become difficult to distribute the denaturant uniformly over the surface of the hydrophilic polymer, thereby making it impossible to attain desired denaturating effect. For example, in case that the hydrophilic polymer and denaturant are an absorbent resin and a crosslinking agent, respectively, an absorbing agent (hydrophilic resin) having an excellent absorbing property under pressure and bonding and shape-keeping properties can not be obtained. Likewise, when less than 1 part by weight of liquid denaturant substance is used, the desired denaturing effect can not be attained, either.
A method of mixing the liquid denaturant substance and water-insoluble compound is not especially limited, and a method of simply mixing the denaturant and water-insoluble compound, a method of mixing a solution or dispersing liquid of the denaturant and the water-insoluble compound, etc. are applicable. In case that a solution or dispersing liquid of the denaturant and the water-insoluble compound are mixed, the solution or dispersing liquid in the denaturant substance is generally sprayed or dropped on the water-insoluble compound first, and thence the solution or dispersing liquid and water-insoluble compound are mixed with each other.
Examples of a mixing apparatus used for mixing the liquid denaturant substance and water-insoluble compound include: a cylindrical mixer, a screw type mixer, a screw type extruder, a turbulizer, a nauter type mixer, a V-shaped mixer, a ribbon type mixer, a two-arm type kneader, a fluidized mixer, a flash type mixer, a rotary disk mixer, a roll mixer, a rotary mixer, etc. The mixing speed can be either high or low. In case that the liquid denaturant substance is a solution or dispersing liquid of the denaturant, water and/or hydrophilic organic solvent are removed when the mixing with the water-insoluble compound ends.
As has been explained, the powdery denaturant substance of the present invention is obtained by mixing the liquid denaturant substance and water-insoluble compound. It is preferable that the powdery denaturant substance is in the form of particles. Generally, a particle size of the powdery denaturant substance is xc2xd or less, and preferably ⅕ or less of the average particle size of the hydrophilic polymer. When the powdery denaturant substance is produced, the kinds or particle size of the water-insoluble compound and mixing conditions of the water-insoluble compound and liquid denaturant substance are selected arbitrary depending on, for example, the intended use of the resulting hydrophilic resin. According to the above arrangement, it has become possible to obtain the powdery denaturant substance which not only denatures the hydrophilic polymer more uniformly, but also further facilitates the control of the denaturation timing.
In addition, in case that the denaturant remains in the liquid state at room temperature, the powdery denaturant substance can be obtained by cooling the liquid denaturant substance to or below a melting point to turn the same into a solid.
A method of mixing the hydrophilic polymer and powdery denaturant substance is not especially limited in the present invention. A mixing apparatus used for mixing the hydrophilic polymer and powdery denaturant substance can be any of the aforementioned example mixing apparatuses. The mixing speed can be either high or low.
According to the present invention, the hydrophilic polymer and denaturant can be mixed homogeneously by the step of mixing the hydrophilic polymer and powdery denaturant substance. Consequently, it has become possible to readily obtain the hydrophilic resin with an excellent balance of properties. In addition, if the heat treatment (heating step) is carried out optionally subsequent to the mixing step to heat the resulting mixture, the hydrophilic polymer and denaturant can react with each other efficiently. Further, if a liquid or gas of an aqueous liquid is added to the resulting mixture before the optional heat treatment and after the mixing step, the hydrophilic polymer and denaturant can also react with each other efficiently. In other words, to denature the hydrophilic polymer with the powdery denaturant substance, the hydrophilic polymer and powdery denaturant substance are mixed with each other, and the hydrophilic polymer and the denaturant contained in the powdery denaturant substance are brought into contact with each other by an adequate means, such as adding an aqueous liquid and/or heating.
A liquid or gas of the aqueous liquid is, for example, water, vapor, a mixed solution of water and a hydrophilic organic solvent, etc. The hydrophilic organic solvent is not especially limited, and can be any of the aforementioned example compounds. In case that a covalent bonding is formed as a result of the reaction of the denaturant and hydrophilic polymer, and for example, when the denaturant is a polyhydric alcohol, polyepoxide, alkylene carbonate or the like, an absorbing agent having a more excellent absorbing property under pressure can be obtained as the hydrophilic resin by adding the aqueous liquid to the mixture of the hydrophilic polymer and powdery denaturant substance. In this case, an amount of water contained in the aqueous liquid varies with a chemical make-up or average particle size of the hydrophilic polymer, a chemical make-up of the powdery denaturant substance, or the intended use of the resulting hydrophilic resin, but a preferable amount with respect to 100 parts by weight of the hydrophilic polymer in solid is 10 parts by weight or less, and a more preferable amount is in a range between 1 part by weight and 5 parts by weight. Likewise, an amount of the hydrophilic organic solvent contained in the aqueous liquid is preferably 10 parts by weight or less, and more preferably in a range between 0.1 part by weight and 5 parts by weight with respect to 100 parts by weight of the hydrophilic polymer in solid. A method of adding the aqueous liquid is not especially limited.
Although a treatment temperature in the heating treatment varies with the kinds of the denaturant or the like, the treatment temperature is preferably 80xc2x0 C. or higher, more preferably in a range between 100xc2x0 C. and 230xc2x0 C., and most preferably in a range between 160xc2x0 C. and 220xc2x0 C. The surface of the hydrophilic polymer is denatured by the heat treatment. When the treatment temperature is below 80xc2x0 C., it becomes difficult to denature the hydrophilic polymer uniformly, and therefore, if the resulting hydrophilic resin is used as an absorbing agent, the absorbing property under pressure of the absorbing agent is hardly improved. In addition, since the heat treatment takes a long time, the productivity of the hydrophilic resin is reduced. The heat treatment is carried out by a typical dryer or furnace. Examples of the dryer include: a channel mixing dryer, a rotary dryer, a disk dryer, a fluidized-bed dryer, a flash type dryer, an infra-red dryer, etc. The heat treatment can be carried out while the hydrophilic polymer and powdery denaturant are being mixed with each other. In short, the mixing step and heat treatment can be carried out in parallel.
As has been explained, the producing method of the hydrophilic resin of the present invention is the method including the mixing step of mixing the hydrophilic polymer and the powdery denaturant substance made from a liquid denaturant substance, and optionally the heating step of heating the mixture obtained in the mixing step. The hydrophilic polymer and liquid denaturant substance are, for example, an absorbent resin containing an acidic group and a crosslinking agent, respectively.
According to the above method, the liquid denaturant substance absorbed in the powdery denaturant substance resides on the surface or inside the water-insoluble compound. For this reason, the liquid denaturant substance can be mixed with the hydrophilic polymer while remaining substantially in the solid state. Thus, according to the above method, the hydrophilic polymer and the liquid denaturant substance, in effect, the powdery denaturant substance, can be mixed with each other more homogeneously compared with the method of mixing the liquid denaturant substance directly with the hydrophilic polymer. The hydrophilic polymer and liquid denaturant substance reside separately in the mixture of the hydrophilic polymer and powdery denaturant substance. In other words, in case that the liquid denaturant substance is made into a powdery denaturant substance, the hydrophilic polymer neither starts to absorb the liquid denaturant substance contained in the powdery denaturant substance nor starts to react with the denaturant contained in the liquid denaturant substance as soon as the hydrophilic polymer and powdery denaturant substance are mixed with each other. The hydrophilic polymer and the denaturant contained in the liquid denaturant substance are brought into contact with each other when the hydrophilic resin has absorbed the aqueous liquid or is heated, thereby making it possible to denature the hydrophilic polymer uniformly.
In case that the powdery denaturant substance is produced by cooling the liquid denaturant substance to or below the melting point, the liquid denaturant substance can be mixed with the hydrophilic polymer while remaining substantially in the solid state. Since the denaturant is contained in the powdery denaturant substance, the hydrophilic polymer and denaturant reside in the hydrophilic resin separately until it is practically used, thereby remaining inactive with each other. When the powdery denaturant substance is fused, the hydrophilic polymer and denaturant are brought into contact and react with each other, whereupon the hydrophilic polymer is denatured (for example, in case that a crosslinking agent is used as the liquid denaturant substance, the surface of the hydrophilic polymer is crosslinked). A method of fusing the powdery denaturant substance is not especially limited, and heat treatment or the like can be adopted.
As has been explained, according to the above producing method, not only the hydrophilic polymer can be denaturated uniformly, but also the denaturation timing can be controlled. For example, according to the above producing method, when the liquid denaturant substance is polyhydric alcohol, polyepoxide, alkylene carbonate, etc., better properties, such as the absorbing property under pressure, can be developed by the denaturation. Particularly, when a crosslinking agent containing a polyamine or the like is used as the denaturant, the bonding ability among the particles of the hydrophilic polymer and the aforementioned shape-keeping ability develop when the hydrophilic resin has absorbed the aqueous liquid during practical use. Further, in this case, the timing of interaction of the particles of the hydrophilic polymer triggered by the polyamine or the like can be controlled. Consequently, the deterioration of the properties, such as gel breaking strength, over time can be prevented more efficiently.
Thus, according to the above arrangement, it has become possible to provide a hydrophilic resin which has an excellent absorbing property under pressure, such as absorbency or water retaining ability under pressure, and shows excellent performance (absorbing property) even when used in the sanitary goods or the like including a high percent by weight of the hydrophilic polymer (resin concentration). In the hydrophilic resin, a liquid introduction space, through which the aqueous liquid migrates to the inside of the hydrophilic polymer, is secured under pressure. Thus, even when 50 wt % or more of the hydrophilic polymer is used, the liquid introduction space can be secured under pressure as well.
Also, according to the above arrangement, it has become possible to provide a hydrophilic resin which has an excellent absorbing property under pressure, prevents the hydrophilic polymer contained therein from releasing from the absorbent material, and maintains its bonding property over time after the production until being put into actual use. The hydrophilic resin contains the hydrophilic polymer and liquid denaturant substance, and when the liquid denaturant substance is the powdery denaturant substance produced by making a crosslinking agent having polyamine or the like into powders, the above-described effect is more significant.
When an absorbing agent is obtained as the hydrophilic resin of the present invention, the absorbing agent is used after being combined with, for example, cellulose fibers, such as paper and comminuted pulp. The absorbing agent has an excellent dispersing ability for the aqueous liquid and does not get wet much. Moreover, since the absorbing agent has an excellent absorbing property, the absorbing agent can be extensively used for sanitary goods, such as paper diapers, sanitary napkins, assisting material for incontinent patients, such as incontinence pads, wound protectors, and wound healing materials, to absorb body fluids; drip absorbing materials to absorb drip from foods or the like, or freshness preserving materials; water-retaining materials for soil to keep water in soil; water stopping materials; etc.